1. Field of the Invention
The present invention relates to a process for producing a high molecular weight aromatic polyamide industrially in an economical and efficient manner.
2. Description of the Prior Art
Aliphatic polyamides such as nylon-6, nylon-66, and nylon-610 have been used as conventional engineering plastics in a broad range of application areas. They are superior in moldability but have a low glass transition temperature, e.g., about 60.degree. C. in the case of nylon-66. This makes them unsuitable for use at high temperatures.
There is another type of polyamide. It includes aromatic polyamides, in which the dicarboxylic acid component is mainly comprised of aromatic dicarboxylic acids. They are superior to aliphatic ones in heat resistance and water resistance but suffer the disadvantage of requiring a high molding temperature because of their high melting point. The problem associated with a high molding temperature is that the cooling rate of a melt injected into a mold is much faster for aromatic polyamide than for aliphatic polyamide. Fast cooling causes the molded articles to have a low crystallinity, which in turn adversely affects stiffness at high temperatures.
To cope with this situation, the present inventors developed a new aromatic polyamide having good heat resistance and mechanical properties. It is composed of repeating units which consist of (a) a dicarboxylic acid component which consists of 60-100 mol % of terephthalic acid and optionally 0-40 mol % of at least one aromatic dicarboxylic acid other than terephthalic acid or straight chain aliphatic dicarboxylic acid of 6-18 carbons and (b) an alkylenediamine component. In addition, the aromatic polyamide has an intrinsic viscosity [.eta.] of 0.5-3.0 dl/g measured in concentrated sulfuric acid at 30.degree. C., as described in Japanese Patent Laid-open No. 53536/1984.
The aromatic polyamide still has a disadvantage due to its high melting point. That is, it is subject to decomposition during production if it is produced in the same manner as the conventional aliphatic polyamides. Therefore, it cannot be produced efficiently.
To address this problem, the present inventors developed a new production process consisting of two steps. The first step is designed to give a low molecular weight condensate having an intrinsic viscosity [.eta.] of 0.05-0.2 dl/g, and the second step is designed to perform melt polymerization of the low molecular weight condensate in an extruder until a desired molecular weight is reached which is indicated by an intrinsic viscosity [.eta.] of 1.0-2.5 dl/g, as described in Japanese Patent Laid-open No. 196625/1985.
A disadvantage of this process is that a long residence time is necessary for melt polymerization of the low molecular weight condensate in an extruder. This leads to the necessity of using an extruder with a high L/D ratio or reducing extrusion outputs. In any way, the process needs a high capacity equipment as well as a large amount of thermal energy.